Chapter 5. Superior and Posterior Mediastina

Big Picture

The mediastinum is the anatomic region medial to the pleural sacs between the sternum, vertebral column, rib 1, and the diaphragm. The mediastinum is further divided into inferior and superior parts by a horizontal plane passing through the sternal angle to the T4–T5 intervertebral disc (Figure 5-1A). The inferior mediastinum is classically subdivided into anterior, middle, and posterior parts. Therefore, the four subregions of the mediastinum are as follows:

• Anterior mediastinum. The region between the sternal angle, the deep sternal surface, the pericardial sac, and the diaphragm. The anterior mediastinum contains fat and areolar tissue and the inferior part of the thymus or its remnant.
• Middle mediastinum. This region contains the pericardial sac and heart (see Chapter 4 for further details).
• Posterior mediastinum. The region containing anatomic structures deep to the pericardial sac, including the thoracic portion of the descending aorta, the azygos system of veins, the thoracic duct, the esophagus, and the vagus and sympathetic nerves (Figure 5-1B). This chapter will focus on the structures located in the posterior mediastinum and their projection into the superior mediastinum.
• Superior mediastinum. The region superior to the sternal angle containing the aortic arch and its three branches, the superior vena cava (SVC) and the brachiocephalic veins, the trachea, the esophagus, and the phrenic and vagus nerves. The superior mediastinum also contains the thymus; however, in an adult, the thymus is usually atrophied and presents as a fatty mass.

Big Picture

The autonomic nervous system of the thorax consists of both sympathetic and parasympathetic motor neurons through which cardiac muscle, smooth muscle, and the glands of the thorax and the abdomen are innervated. Autonomic innervation involves two types of neurons: preganglionic neurons and postganglionic neurons. The sympathetic nerves in the thorax and in other areas of the body include visceral sensory fibers that course along the general sensory neurons.

Sympathetic Nerves of the Thorax

The thoracic sympathetic chain or trunk courses vertically across the heads of the ribs along the posterior thoracic wall, deep to the parietal pleura (Figure 5-2A). This location parallels the vertebral column; the sympathetic chain, therefore, is also referred to as the paravertebral ganglia. The sympathetic chain descends posterior to the diaphragm to continue its descent in the abdominal cavity.

The thoracic portion of the sympathetic trunk typically has 12 paravertebral ganglia connected to adjacent thoracic spinal nerves by white and gray rami communicantes. White rami have myelinated nerve fibers and thus appear white. Gray rami have unmyelinated nerve fibers and, therefore, appear gray. The sympathetic ganglia are numbered according to the thoracic spinal nerve with which they are associated. The chain is composed primarily of ascending and descending preganglionic sympathetic fibers and visceral afferent fibers. Each paravertebral ganglion houses the nerve cell bodies for postganglionic sympathetic nerve fibers. Internodal fibers connect sympathetic ganglia vertically, resulting in the chain or trunk.

The cell bodies for preganglionic sympathetic fibers originate in the lateral horn of the spinal cord gray matter. The axons exit the ventral root into the ventral ramus where white rami convey the preganglionic sympathetic fibers from the ventral ramus to a paravertebral ganglion. Once the preganglionic sympathetic fibers enter the paravertebral ganglion, the following possible pathways may occur (Figure 5-2B):

1. Preganglionic sympathetic fibers synapse with postganglionic sympathetic fibers within the ganglion, and the segmental gray rami communicantes carry the postganglionic sympathetic fibers back to the ventral ramus at the same vertebral level. The postganglionic sympathetic fibers innervate blood vessels, sweat glands, and arrector pili muscles of hair follicles within the associated dermatome.

2. Preganglionic sympathetic fibers enter internodal fibers, ascending to a higher ganglion (e.g., the superior cervical ganglion), where they synapse with a postganglionic fiber. Postganglionic sympathetic fibers exit the ganglion en route to the cardiac plexus (see Chapter 4, Innervation of the Heart).

3. Preganglionic sympathetic fibers en route to abdominal organs become thoracic splanchnic nerves. Thoracic splanchnic nerves are formed from the following levels (Figure 5-2A):

   • T5–T9 unite to form the greater splanchnic nerve, which perforates the crus of the diaphragm or occasionally passes through the aortic hiatus and ends in the celiac or superior mesenteric ganglion.
   • T10–T11 unite to form the lesser splanchnic nerve, which pierces the crus of the diaphragm and ends in the aorticorenal ganglion.
   • T12 becomes the least splanchnic nerve, which pierces the crus of the diaphragm and ends in the renal plexus.

Big Picture

The structures superficial to the sympathetic chain are the azygos system of veins (which drain the posterior thoracic and abdominal walls), the thoracic lymphatic duct (which courses between the thoracic aorta and esophagus), and the thoracic aorta (located left of the midline, along the anterior surface of the thoracic vertebrae).

Azygos System of Veins

The azygos system of veins is formed through the convergence of the following unpaired veins (Figure 5-3):

• Azygos vein. The right ascending lumbar and right subcostal veins unite to form the azygos vein and ascend into the thorax through the aortic opening of the diaphragm. The azygos vein receives blood directly from the right posterior intercostal veins and indirectly via the left-sided connections from the hemiazygos and accessory hemiazygos. The azygos vein terminates by coursing anteriorly at the T4 vertebral, to arch over the right primary bronchus and converge with the superior vena cava.
• Hemiazygos vein. The union of the left ascending lumbar and left subcostal veins forms the hemiazygos vein. It ascends on the left side of the vertebral bodies, posterior to the thoracic aorta, and receives the lower four posterior intercostal veins. The hemiazygos vein joins the azygos vein via tributaries that cross the vertebral column from left to right. The hemiazygos vein often communicates with the left renal vein.
• Accessory hemiazygos vein. The accessory hemiazygos vein begins at the fourth intercostal space and receives posterior intercostal veins four through eight. It descends anterior to the posterior intercostal arteries and joins the azygos vein.

The first posterior intercostal vein on each side drains into the corresponding brachiocephalic vein. Posterior intercostal veins two through four join to form the superior intercostal vein, which drains into the azygos vein on the right side and the brachiocephalic vein on the left side.

Lymphatic Drainage

Lymph in the thoracic region drains via two lymphatic vessels, the thoracic duct and the right lymphatic duct.

1. The thoracic (lymphatic) duct

   • Is the main lymphatic channel that receives lymph from the entire body, with the exception of the right upper limb, the right side of the head and neck, and the right upper thorax.
   • May have a beaded appearance because of its numerous valves.
   • Begins in the abdomen just inferior to the diaphragm at a dilated sac created by convergence of the intestinal and lumbar lymphatic trunks, called the cisternal chili.
   • Ascends deep to the diaphragm by coursing through the aortic hiatus.
   • Continues superiorly between the esophagus and thoracic aorta.
   • Shifts to the left of the esophagus at the level of the sternal angle (T4 vertebral level).
   • Curves laterally at the root of the neck and empties into the junction of the left internal jugular and subclavian veins.

2. The right lymphatic duct

   • Drains the right side of the thorax, the upper limb, and the head and neck and empties into the junction of the right internal jugular and subclavian veins.

Thoracic Aorta

The thoracic (descending) aorta begins at the T4 vertebral level and courses anterior to the vertebral column, just left of the midline. The thoracic aorta enters the abdominal cavity through the aortic hiatus at vertebral level T12. The following branches arise from the thoracic aorta:

• Paired posterior intercostal arteries (3–11); the first two intercostal arteries arise from the costocervical trunk.
• Bronchial arteries (usually one to the right bronchus and two to the left bronchus) for supply of the nonrespiratory tissues in the lungs.
• Multiple esophageal branches to the middle third of the esophagus.
• Paired subcostal arteries.
• Paired superior phrenic arteries to the posterior regions of the diaphragm.

Big Picture

The esophagus is a muscular tube that is continuous with the pharynx in the neck and enters the thorax posterior to the trachea. The esophagus is located against the upper thoracic vertebrae in the midline and descends to the left of the aorta (near vertebral level T4). Because of its location deep to the pericardium, the esophagus has its anterior surface pressed by the left atrium. At vertebral level T10, the esophagus exits the thorax through the esophageal hiatus of the diaphragm. Note that the esophageal hiatus is formed by the right crus of the diaphragm, splitting to wrap around the esophagus (the so-called esophageal sphincter).

Vasculature of the Esophagus

The esophagus receives its blood supply in the thorax from branches of the thoracic aorta (e.g., the esophageal and bronchial arteries). Additional contributing arteries include the left gastric and inferior phrenic arteries.

Vagus Nerve

The right and left vagus (parasympathetic) nerves supply the esophagus via the esophageal plexus. Preganglionic parasympathetic fibers originate bilaterally from the medulla oblongata and travel through the vagus nerve through the thorax, posterior to the root of the lung (Figure 5-4).

• Right vagus nerve. Descends into the thoracic cavity anterior to the right subclavian artery and gives rise to the right recurrent laryngeal nerve, which hooks around the right subclavian artery and ascends back into the neck en route to intrinsic laryngeal muscles. The right vagus nerve continues inferiorly by coursing posterior to the superior vena cava and right primary bronchus and curves to the posterior surface of the esophagus, becoming the posterior vagal trunk. The right vagus nerve also contributes to the cardiac and pulmonary plexuses.
• Left vagus nerve. Enters the thorax between the left common carotid and subclavian arteries and gives rise to the left recurrent laryngeal nerve, which hooks around the aortic arch by the ligamentum arteriosum and ascends back into the neck en route to the intrinsic laryngeal muscles. The left vagus nerve continues on to the anterior surface of the esophagus, becoming the anterior vagal trunk. The left vagus nerve also contributes to the cardiac and pulmonary plexuses.

The anterior and posterior vagal trunks exchange fibers, creating an esophageal plexus (web) of nerves. The vagus nerve carries visceral sensory fibers whose sensory cell bodies are located in the inferior vagal ganglion. The visceral afferents from the vagus nerves transmit information to the brain about normal physiologic processes and visceral reflexes. They do not relay pain information.

Big Picture

The superior mediastinum is a thoroughfare for vessels, nerves, and lymphatics to pass between the neck, upper limbs, and thorax. The trachea, bronchial tree, pulmonary arteries, and aortic arch are important for oxygen transport and exchange from the external environment to internal structures.

Trachea and Bronchial Tree

The trachea begins at the cricoid cartilage, at the C6 vertebral level, and has 18 to 20 incomplete hyaline cartilaginous rings, which are open posteriorly (Figure 5-5). The cartilaginous rings prevent the trachea from collapsing while a person is exhaling. The trachea is anterior to the esophagus and bifurcates into the right and left primary (principal or mainstem) bronchi at the level of the sternal angle (T4–T5 vertebral level). The point of bifurcation, called the carina, is marked in the inside of the airway by a cartilaginous wedge that projects upward into the lumen of the airway.

• Right primary bronchus. Is shorter, wider, and more vertical than the left primary bronchus. The right primary bronchus courses inferior to the arch of the azygos vein and divides into superior, middle, and inferior secondary (lobar) bronchi.
• Left primary bronchus. Crosses anterior to the esophagus and divides into two secondary (lobar) bronchi: the superior and inferior lobar bronchi.

Each lobar bronchus further divides into segmental (tertiary) bronchi, which further divide. The smallest bronchi give rise to bronchioles, which terminate in alveolar sacs, where the exchange of gases takes place.

Pulmonary Arteries

The pulmonary trunk and pulmonary arteries are located approximately at vertebral level T4, where the arteries are inferior to the aortic arch and azygos vein. The pulmonary arteries follow the bronchial tree throughout the lungs. The relation of the pulmonary arteries to bronchi at the root of the lungs can be remembered by the following mnemonic: Right pulmonary artery is Anterior to the right primary bronchus, and the Left pulmonary artery is Superior to the left primary bronchus (RALS).

Aortic Arch and Associated Branches

The ascending aorta arises from the heart at the T4 vertebral level and ascends into the superior mediastinum over the pulmonary vessels and left primary bronchus to become the aortic arch. A fibrous connective tissue cord, called the ligamentum arteriosus, connects the deep surface of the aortic arch to bifurcation of the pulmonary trunk in the adult. The ligamentum arteriosus is a remnant of the ductus arteriosus, which, during fetal development, shunted blood from the pulmonary trunk to the aorta, similar to the patent foramen ovale. The aortic arch terminates at the T4 vertebral level to become the thoracic (descending) aorta.

The aortic arch has the following three branches (Figure 5.5):

• Brachiocephalic artery. The first branch of the aortic arch. The brachiocephalic artery courses superiorly to the right, where it bifurcates into the right common carotid and right subclavian arteries, supplying the right side of the head and neck and upper limb, respectively.
• Left common carotid. The second branch of the aortic arch. The left common carotid artery supplies the left side of the head and neck.
• Left subclavian arteries. The third branch of the aortic arch. The left subclavian artery supplies the left upper limb.